Polyisoprene articles and process for making the same

ABSTRACT

The invention disclosed herein relates to an improved process for making elastomeric polyisoprene articles. In particular, the process of the invention is a system which produces synthetic polyisoprene articles exhibiting tensile strength properties similar to that of solvent-based processes using natural rubber latex. The process comprises an accelerator composition at the pre-cure stage comprising a dithiocarbamate, a thiazole and a guanidine compound. In a preferred embodiment, the accelerator composition comprises zinc diethyldithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT) and diphenyl guanidine (DPG), in conjunction with a stabilizer, such as sodium caseinate. The invention also includes an elastomeric polyisoprene product made by the process, such as a surgeon&#39;s glove.

RELATED APPLICATION DATA

This application is based on U.S. provisional patent application Ser.No. 60/275,087 filed on Mar. 12, 2001.

FIELD OF THE INVENTION

The invention relates to the field of elastomeric articles used in themedical field. In particular, the invention relates to improvements tothe process of making elastomeric polyisoprene articles for medicalapplications.

BACKGROUND OF THE INVENTION

The manufacturing process for producing elastomeric articles fromnatural or synthetic rubber latex involves a curing step during whichcross-linking or vulcanization through sulfur groups occurs between thepolymer units. Conventional processes for making elastomeric articlesfrom natural or synthetic latex typically involve preparing a latexdispersion or emulsion, dipping a former in the shape of the article tobe manufactured into the latex and curing the latex while on the former.Desirable properties of certain elastomeric articles such as tensilestrength are substantially affected by the cross-linking and curingstages of the manufacturing process.

The use of vulcanizing or sulfur cross-linking accelerator compounds inthe manufacture of rubber articles is well-known. Conventionalvulcanization accelerators include dithiocarbamates, thiazoles,guanidines, thioureas, amines, disulfides, thiurams, xanthates andsulfenamides. The use of vulcanization accelerators in the manufactureof polyisoprene rubber is disclosed in D'Sidocky et al., U.S. Pat. No.5,744,552 and Rauchfuss et al., U.S. Pat. No. 6,114,469. Certain fieldsin which elastomeric articles are needed, such as the medical field,utilize specific types of equipment and processing techniques whichaccommodate the specific performance and regulatory requirements of theparticular article produced.

The use of natural rubber latex in the manufacture of certain articlessuch as medical gloves has been associated with disadvantageousproperties, such as allergic reactions believed by some to be caused bynatural proteins or allergens present within the natural rubber latexand the final product. Of increasing interest in the medical field,particularly in the field of gloves, are synthetic elastomeric productsand manufacturing processes which altogether reduce, or altogetheravoid, the likelihood of potential adverse reactions of the user orwearer.

Synthetic elastomeric polyisoprene articles such as gloves are known andare of interest in the art as an alternative to the use of naturallatex. Commercially available synthetic gloves include those elastomerscomposed of polychloroprene (neoprene), carboxylated acrylonitrilebutadiene (nitrile),styrene-isoprene-styrene/styrene-ethylene-butylene-styrene blockco-polymers, polyurethane, and polyisoprene. Polyisoprene is one of themost preferred polymers due to its chemical similarity to natural rubberas well as its physical properties such as feel, softness, modulus,elongation and tensile strength. One such polyisoprene glove iscommercially available from Maxxim Medical (Clearwater, Fla.).

A majority of glove manufacturing processes are water-based dippingsystems. It is known that solvent-based systems are possible forpolyisoprene, although such systems are poorly suited for themanufacture and molding of elastomeric articles for medicalapplications. One difficulty in the field of gloves, for example, is thedesign of processes and materials which will produce a thin elastomericarticle having desirable properties such as high tensile strength.Another disadvantage of solvent-based systems is solvent toxicity.Process and materials which would obviate or reduce the need for the useof toxic solvents while at the same time yielding a product havingdesirable properties for medical applications are thus still beingexplored.

Accordingly, there exists a need in the medical device field forimproved manufacturing processes for making synthetic elastomericarticles. Especially desirable would be processes which can producepolyisoprene articles, such as surgical gloves, which possess thedesirable properties found in the natural rubber counterpart, while atthe same time permitting economical and cost-effective manufacturing.

SUMMARY OF THE INVENTION

Applicants have discovered a three-part accelerator composition forsulfur cross-linkable polyisoprene latex which can be used with latex ina process for making elastomeric articles having the desirableproperties (e.g., tensile strength) similar to that of natural rubberbut without the presence of natural rubber latex proteins and allergens.Another advantage is that the accelerator system is suitable for medicalapplications where thin molded elastomeric articles are required, suchas gloves. Furthermore, the accelerator composition and process of theinvention permits the use of a solvent-free, water-based process system,as opposed to a solvent-based process system. The resultant article hasproperties similar to those produced using the solvent-based system.Accordingly, the use of solvents can be reduced or avoided and solventtoxicity can likewise be avoided using the invention.

Another advantage of the invention is that conventional manufacturingequipment and most readily-available materials can be used in accordancewith the invention to make the synthetic polyisoprene glove without theneed for new or costly additional materials or equipment. Further, nocomplicated new process steps are required by the invention and theinvention can be readily incorporated into existing glove makingprocesses and systems.

Another aspect of the invention is that the compounded (or ready to use)polyisoprene latex composition formulated in accordance with theinvention exhibits prolonged storage stability. For example, thepre-cure storage stability of the compounded polyisoprene latexcomposition (i.e., the time period prior to the use of the compoundedpolyisoprene latex composition in the dipping and curing stages) canextend up to about 8 days, in contrast to the typical current 3 to 5 daytime period. By extending storage life of the latex, the amount ofwasted latex can be significantly reduced and greater flexibility inscheduling manufacturing processes is permitted.

Yet another advantage is that the process of the invention allows forsignificantly reduced pre-cure process parameters (lower temperature andshorter time periods than conventionally used) and lower dippingtemperatures in the manufacturing process. Accordingly, significant costand resource advantages are provided over conventional manufacturingpractices.

The invention provides for a process of making a synthetic elastomericpolyisoprene article comprising the steps of: a) preparing a compoundedpolyisoprene latex composition containing an accelerator compositioncontaining a dithiocarbamate, a thiazole and a guanidine compound; b)dipping a former into said compounded polyisoprene latex composition;and c) curing said compounded polyisoprene composition on said former.Additionally, the initial pre-cure processing (i.e., prior to storageand article manufacture) can be performed at temperatures of less than35° C. and in time periods as short as ranging from about 90 minutes(1.5 hours) to about 150 minutes (2.5 hours), preferably about 120minutes (2.0 hours). The compounded polyisoprene latex composition canbe stored for periods up to about 8 days at ambient temperatures(ranging from about 15° C. to about 20° C.). Lower temperatures can beused for the latex dipping step as well

The invention also provides for a synthetic elastomeric polyisoprenearticle made by a process comprising the steps of: a) preparing acompounded polyisoprene latex composition comprising an acceleratorcomposition comprising a dithiocarbamate, a thiazole and a guanidinecompound; b) pre-curing said compounded polyisoprene latex compositionc) dipping a former into said compounded polyisoprene latex composition;and d) curing said compounded polyisoprene composition on said former.Elastomeric articles made by the process of the invention can exhibittensile strengths of over 3000 psi (as measured in accordance with ASTMD412) even after as much as 7 days of latex storage prior to use in thearticle manufacturing process.

The invention further provides for a synthetic polyisoprene latexcomposition comprising:

polyisoprene latex;

a dithiocarbamate compound;

a thiazole compound; and

a guanidine compound.

The invention also provides for an accelerator composition for use inmaking elastomeric polyisoprene articles consisting essentially of:

a dithiocarbamate compound;

a thiazole compound;

a guanidine compound;

wherein the phr (parts per hundred) dry weight ratio of each of thedithiocarbamate; thiazole; and guanidine ranges from about 0.50 to about1.00 per 100.0 parts polyisoprene.

In a preferred embodiment, the accelerator composition comprises zincdiethylthiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT) anddiphenyl guanidine (DPG) and used in conjunction with a stabilizer.Preferably, the stabilizer is an alkali earth metal case mate salt, suchas sodium caseinate.

DETAILED DESCRIPTION OF THE INVENTION

The accelerator composition of the invention can be used in conjunctionwith conventional equipment and materials otherwise known to be used inthe manufacture of elastomeric articles composed of polyisoprene. Ingeneral, the process begins with the preparation of the compoundedpolyisoprene latex composition. The synthetic polyisoprene latex iscombined with the accelerator composition, a stabilizer, and additionalingredients to prepare the polyisoprene latex composition in accordancewith the invention. The function of the accelerator is to increase therate of vulcanization, or the cross-linking of polyisoprene to enhancethe curing properties of the latex during the curing stages of theprocess. Prior to the dipping and curing steps, the compounded latexincluding the accelerator composition can be used immediately or storedfor a period of time prior to its employment in the dipping process.

When the compounded polyisoprene latex composition is ready for use orfollowing storage, a former in the overall shape of the article to bemanufactured is first dipped into a coagulant composition to form acoagulant layer directly on the former. Next, the coagulant-coatedformer is dried and then dipped into the compounded polyisoprene latexcomposition.

The latex-covered former is then subjected to the curing step. The latexis cured directly onto the former at elevated temperatures therebyproducing an article in the shape of the former. Further steps aretypically performed as well, such as leaching with water, beading thecuff, and the like. These techniques are well-known in the art.Additional post-treatment processes and techniques steps are oftenperformed as well, such as lubrication and coating, halogenation (e.g.,chlorination), and sterilization.

A variety of elastomeric articles can be made in accordance with theinvention. Such elastomeric articles include, but are not limited to,medical gloves, condoms, probe covers (e.g., for ultrasonic ortransducer probes), dental dams, finger cots, catheters, and the like.As the invention provides numerous advantages and benefits in a numberof ways, any form of elastomeric article which can be composed ofpolyisoprene can benefit from the use of the invention.

Polyisoprene latex is the major component of the pre-cure latexcomposition. Suitable polyisoprene latex which can be used is readilyavailable and can be obtained from a number of commercial sources,including but not limited to, Kraton™ Corporation, Houston, Tex.; ShellInternational Corporation, Houston, Tex.; Apex Medical Technologies,Inc. San Diego, Calif.; and Aqualast™ E0501 available from LordCorporation, Erie, Pa. In addition to polyisoprene, polyisopreneco-polymers and polyisoprene blends can be used as well. Polyisopreneco-polymers which can be used include any co-polymer having an isoprenemonomer unit and having sufficiently similar chemical structural andproperties of polyisoprene to exhibit the desirable properties of thepolyisoprene product when combined with the accelerator composition andmade according to the process of the invention. Suitable polyisopreneblends can include, but are not limited to: natural rubber latex;polydiene and its co-polymers, such as polybutadiene; substitutedpolydiene, such as polychloroprene; thermoplastic materials, such aspolyurethane; and the like.

The accelerator composition of the invention comprises at least onedithiocarbamate, at least one thiazole, and at least one guanidinecompound. Preferably, the dithiocarbamate compound for use with theinvention is zinc diethyldithiocarbamate, also known as ZDEC or ZDC.Suitable ZDEC which can be used includes Bostex™ 561 (commerciallyavailable from Akron Dispersions, Akron, Ohio). The preferred thiazolecompound for use in the invention is zinc 2-mercaptobenzothiazole, alsoknown as zinc dimercaptobenzothiazole or ZMBT. Suitable ZMBT which canbe used includes Bostex™ 482A (commercially available from AkronDispersions, Akron, Ohio). In a preferred embodiment, the guanidinecompound used in the accelerator composition is diphenyl guanidine, alsoknown as DPG. Suitable DPG which can be used includes Bostex™ 417(commercially available from Akron Dispersions, Akron, Ohio).

Other dithiocarbamate, thiazole and guanidine derivatives can also beuse in accordance with the invention, provided each is chemicallycompatible with, i.e., does not substantially interfere with thefunctionality of, the remaining two accelerator compounds used.Dithiocarbamate derivatives which can also be used include zincdimethyldithiocarbamate (ZMD), sodium dimethyldithiocarbamate (SMD),bismuth dimethyldithiocarbamate (BMD), calcium dimethyldithiocarbamate(CAMD), copper dimethyldithiocarbamate (CMD), leaddimethyldithiocarbamate (LMD), selenium dimethyldithiocarbamate (SEMD),sodium diethyldithiocarbamate (SDC), ammonium diethyldithiocarbamate(ADC), copper diethyldithiocarbamate (CDC), lead diethyldithiocarbamate(LDC), selenium diethyldithiocarbamate (SEDC), telluriumdiethyldithiocarbamate (TEDC), zinc dibutyldithiocarbamate (ZBUD),sodium dibutyldithiocarbamate (SBUD), dibutyl ammoniumdibutyldithiocarbamate (DBUD), zinc dibenzyldithiocarbamate (ZBD), zincmethylphenyl dithiocarbamate (ZMPD), zinc ethylphenyl dithiocarbamate(ZEPD), zinc pentamethylene dithiocarbamate (ZPD), calciumpentamethylene dithiocarbamate (CDPD), lead pentamethylenedithiocarbamate (LPD), sodium pentamethylene dithiocarbamate (SPD),piperidine pentamethylene dithiocarbamate (PPD), and zinc lopetidenedithiocarbamate (ZLD).

Other thiazole derivatives which can be used include2-mercaptobenzothiazole (MBT), copper dimercaptobenzothiazole (CMBT),benzthiazyl disulphide (MBTS), and 2-(2′,4′-dinitrophenylthio)benzthiazole (DMBT).

Other guanidine derivatives which can be used include diphenyl guanidineacetate (DPGA), diphenyl guanidine oxalate (DPGO), diphenyl guanidinephthalate (DPGP), di-o-tolyl guanidine (DOTG), phenyl-o-tolyl guanidine(POTG), and triphenyl guanidine (TPG).

The proportions and ratios of the ingredients of the acceleratorcomposition can vary somewhat provided all three of the ingredients,i.e., dithiocarbamate, thiazole and guanidine compounds, are present.With respect to the preferred accelerator ingredients, each of theaccelerator compounds zinc diethyldithiocarbamate (ZDEC), zinc2-mercaptobenzothiazole (ZMBT) and diphenyl guanidine (DPG) can bepresent in an individual amount ranging from about 0.50 phr (parts byweight per 100 parts by weight of rubber) to about 1.00 phr dry weightper 100 parts polyisoprene. In other words, the accelerator compositionsof the invention comprise ZDEC:ZMBT:DPG phr dry weight ratios rangingrespectively from about 0.50:0.50:0.50 phr to about 1.00:1.00:1.00 phr.

In a preferred embodiment, a stabilizer is used in conjunction with theaccelerator composition. Any stabilizer known in the art useful incurable latex systems can be used provided it is chemically compatiblewith the other ingredients and provides the desired function, i.e.,prolongs stabilization of the pre-cure compounded polyisoprene latex. Avariety of stabilizers can be used, including but not limited to, milkprotein salts, anionic surfactants such as sodium lauryl sulfates, andsorbitan fatty acid esters.

Milk protein salts are preferred for use as the stabilizer. Inparticular, alkali earth metal caseinate salts are preferred. Alkaliearth metal caseinate salts which can be used in accordance with theinvention include, but are not limited to, sodium caseinate, potassiumcaseinate, manganese caseinate and zinc caseinate, and combinationsthereof. Most preferred for use as the stabilizer is sodium caseinate(commercially available from Technical Industries, Inc., Peacedale,R.I.).

Anionic surfactants which can be used as stabilizers for the inventioninclude Rhodopex® ES (a composition having a sodium lauryl (3) sulfateactive available from Rhodia, Cranbury, N.J.) and Rhodacal® DS-10 (acomposition having a branched sodium dodecylbenzene active availablefrom Rhodia, Cranbury, N.J.). Sorbitan fatty acid ester surfactantswhich can be used as stabilizers in the invention includepolyoxyethylene sorbitan fatty acid esters such as Tween® 80 (apolysorbate available from ICI Americas, Inc., Wilmington, Del.).

The amount of stabilizer present in the pre-cure polyisoprene latexcomposition is preferably ranges from about 0.50 phr dry weight to about1.00 phr dry weight (per 100.00 parts dry weight polyisoprene).Preferably, the amount of stabilizer is present in an amount of about0.75 phr dry weight.

In addition to the polyisoprene, accelerator composition and stabilizer,additional ingredients which enhance or facilitate the manufacturingprocess can be included in the compounded polyisoprene latex compositionas well. The compounded polyisoprene latex composition can also includecatalysts (or accelerator initiators) such as alkali earth metal oxidesand methyl oxides, preferably zinc oxide (ZnO) (commercially availablefrom Maxxim Medical, Eaton, Ohio); curing (or cross-linking) agents suchas elemental Sulfur (e.g., Bostex™ 378 commercially available from AkronDispersion, Akron, Ohio), organic sulfides or other sulfur donorcompounds; and anti-oxidants, such as Wingstaym™ L (e.g., butylatedreaction product of p-cresol and dicyclopentadiene (DCPD) such asBostex™ 24 available from Akron Dispersion, Akron, Ohio).

Preparation of Polyisoprene Latex Composition

The compounded polyisoprene latex composition in accordance with theinvention can be prepared using the following general procedure:

Polyisoprene latex (typically 60% solids) and the stabilizer (e.g.,sodium caseinate) are combined at ambient temperature (about 20° toabout 25° C.). After mixing for a period of time, the mixture is thendiluted to 40% solids in water. Wingstay L is then added and the mixtureis stirred for approximately 15 minutes. At this point, the pH can beadjusted to a range of about 8.5 to 9.0. Zinc oxide is added, followedby the sulfur and accelerator compounds. Preferred accelerator compoundsare ZDEC, ZMBT and DPG and are added in ratios ranging from0.50:0.50:0.50 phr to 1.00:1.00:1.00 phr dry weight per 100.0 partspolyisoprene. The mixture is then heated to a temperature within a rangeof about 20° C. to about 40° C., preferably from about 25° C. to about30° C., while continuously stirring for a time period ranging from about1.5 hours to about 2.5 hours, preferably about 2 hours, using a magneticstirrer and heating plate.

The mixture is then cooled to a temperature ranging of less than about25° C., typically ranging from about 15° C. to about 20° C. Thecompounded latex is preferably stored at ambient temperatures rangingfrom about 15° to about 20° C. At these temperatures, the compoundedpolyisoprene latex composition can be stored for periods lasting up toabout 8 days prior to its use in the dipping and curing process.

Preparation of a Polyisoprene Glove

Initially, the pH of the compounded polyisoprene latex can be adjustedto a pH of approximately 10. A glove former is pre-heated in an oven toa temperature of about 70° C. and then dipped in a pre-preparedcoagulant composition at a temperature of about 55° C. for a period oftime and then removed therefrom. Next, the coagulant-coated former isplaced in a drying oven at 70° C. for a time sufficient to dry thecoagulant, typically about 5 minutes.

The coagulant-coated former is removed from the oven and dipped into thecompounded polyisoprene latex at ambient temperature, or a temperatureranging from about 20° C. to about 25° C. The coated former is removedand placed in oven at a temperature of about 70° C. for about 1 minute.The glove and former are removed from oven and placed into waterleaching tank having a temperature of about 65° C. for about 5 minutes.The glove and former are removed from the leaching tank and placed driedat about 70° C. for a period sufficient to dry the glove, typicallyabout 5 minutes. This is the end of the first curing stage.

At the second curing stage, the glove and former are placed in an ovenheated to a temperature of about 120° C. for about 20 minutes. The gloveand former are removed and cooled to ambient temperature. Finally, theglove is stripped from the former.

The gloves can be further treated in accordance with the particularneeds, such as using lubrication, coating, halogenation, andsterilization techniques, all of which are conventional. Otherconventional steps can be incorporated into the general process as well.

When prepared in accordance with the invention, elastomeric articlessuch as gloves exhibit the following physical properties: tensilestrength of greater than about 3000 psi, elongation of greater thanabout 750% at break, and a tensile modulus of less than about 300 psi at300% elongation as measured in accordance with ASTM D412.

Other elastomeric polyisoprene articles can be prepared using processessimilar to those described herein, in combination with conventionalequipment and techniques readily available in the art. For example, anelastomeric article in the form of condom can be prepared using a condomformer.

The following example further illustrates the advantages of theinvention and should not be construed as limiting the invention to theembodiments depicted therein.

EXAMPLES Example 1 Preparation of a Polyisoprene Glove

Polyisoprene latex (Kraton™ IR PR401 lot # 000313 having TSC 64.40%obtained from Shell International Corporation, Houston, Tex.) wasdiluted with water. Sodium caseinate (obtained from TechnicalIndustries, Inc., Peacedale, R.I.) was then added to the mixture andstirred at ambient temperature. While under continuous stirring, zincoxide and sulfur dispersions were added to the mixture. Acceleratorcompounds ZDEC (from Akron Dispersions, Akron Ohio), ZMBT, and DPG (fromAkron Dispersions, Akron, Ohio) were formulated into dispersions andthen added. Wingstay™ L was added and the mixture was stirred forapproximately 15 minutes. The composition was diluted to about 37.0%solids with water. The pH was adjusted using ammonium hydroxide to pH10.7. The composition was maintained at a temperature of 25° C. andstored under continuous agitation for 24 hours at a temperature of lessthan 25° C.

Accordingly, the following is a summary of the formulation ingredientsand their respective amounts. All percentages are percentages by weightunless otherwise noted.

Latex Formulation:

Ingredient Parts (phr) dry weight Polyisoprene 100.00 ZDEC 0.50 ZMBT0.50 DPG 1.00 Sodium caseinate 0.75 ZnO 0.50 Sulfur 1.25 Wingstay ™ L2.00

A glove former was preheated to 100° C. in an oven, removed and dippedinto a coagulant composed of soft water 80.65%, calcium nitrate 13.65%,calcium carbonate 5.46%, wetting agent (Surfonyl™ TG 0.2%), cellulose(Cellosize™ QP 52000) 0.04%) at a temperature of 56° C. for a period of30 seconds and removed. The coagulant-coated former was cooled to atemperature of about 58° C. and was placed in a drying oven at atemperature of 100° C. for a period of time sufficient to dry thecoagulant.

The coagulant-coated former was removed from the oven and dipped intothe compounded polyisoprene latex composition of Formula 1 at atemperature of 25° C. for a period of 0.8 minutes. The coated former wasremoved and placed into a pre-heated oven at a temperature of 130° C.for a period of 0.8 minutes.

The coated former was then removed from the oven and placed into waterleaching tank at a temperature of 50° C. for a period of 5.0 minutes.The former was removed from the leaching tank and placed into an oven ata temperature of 70° C. for 30 seconds.

The former was removed from the oven and dipped into a silicone tank ata temperature of 40° C. for 30 seconds. The former was removed from thesilicon tank and while still on the former, the glove was beaded at thecuff using a beader roller.

The former were then placed into a second stage cure oven and movedtherethrough at zone temperatures ranging from 110° C. to 135° C. for atotal time period lasting for a period of 9.5 minutes. After exiting thecuring oven, the glove was subjected to a post-cure leaching. At thisstep, the glove on the former was rinsed with water at a temperature of70° C. water for a period of about 1 minute.

The glove was placed in a slurry tank at a temperature of 55° C. for 30seconds. The slurry composition contained 85.2% water, 14.33% starch,0.4% cellulose (Cellosize™ QP 52000), 0.4% sodium hypochlorite, 0.01%surfactant (Darvan™), and 0.02% Casastab™ T. The formers were thenplaced into a post-slurry oven to dry the glove thereby producing thefinal glove. The glove covered former was cooled and the glove wasstripped therefrom.

The physical properties of the glove produced by the above process wereevaluated. Samples were obtained from the gloves exhibited averagetensile strength values of 3810 psi, tensile modulus value of 171 psi at300% elongation, and 1125% elongation at break as measured using ASTMD142.

Example 2 Comparative Data Using Different Accelerator Formulations andProcess Conditions

Differing compounded polyisoprene latex compositions and varying processparameters were used to prepare samples, the physical properties ofwhich were then tested and evaluated. Compounded latex containingvarious accelerator compounds and phr (parts per hundred) ratios wereprepared in accordance with a process similar to that of Example 1Process for Preparation of Polyisoprene Latex Composition”, andpre-cured and stored at the corresponding temperatures and conditionsdescribed or listed in Table 1 below.

Test samples were prepared from compounded latex formulations at variousintervals over a total latex storage period of eight (8) days. Each ofsamples 1 and 3 through 16 were then prepared by heating plates to atemperature of about 70° C. for a period of about 5 minutes, andsubsequently dipping the plates in coagulant (35% calcium nitrate, 7%calcium carbonate, 0.03% Surfonyl™ TG) at a temperature of about 55° C.for a period of about 10 seconds. The coagulant coated plates were thendried at 70° C. for a period of about 5 minutes. The coated plates werethen dipped into the compounded polyisoprene compositions, which werestored and dipped at the corresponding temperature shown in Table 1. Theplates were leached with water at a temperature of about 65° C. for aperiod of about 3 minutes, and subsequently dried at a temperature ofabout 70° C. for a period of about 5 minutes. The plates were then curedat a temperature of 120° C. for a period of about 20 minutes. Thesamples were then stripped from the plates.

Samples 2a and 2b were prepared using slightly different processparameters and were obtained from articles prepared usingmanufacturing-scale parameters and equipment. For each of samples 2a and2b, a mold (glove former) was heated to a temperature of about 55° C.and dipped in coagulant (same coagulant as above) at a temperature ofabout 55° C. The coagulant-covered mold was then dried in an oven at atemperature of about 70° C. for a period of about 3 minutes. The driedcoagulant-coated mold was removed from the oven and dipped into thecompounded latex composition for a period of about 12 seconds dwellingtime, removed for a period of about 6 seconds unsubmerged, and thenredipped for a further 8 seconds. The latex-coated mold was leached attemperature of about 50° C. for a period of about 5 minutes, andsubsequently cured at a temperature of about 135° C. for a period ofabout 15 minutes.

The following Table I is a summary of the process parameters andcompounded latex formulations prepared:

TABLE 1 Accelerator and Stabilizer Formulations and Process ConditionsAccelerator Composition (ZDEC/ZMBT/ Stablizer Sample No. DPG Pbr ratio)(type/phr) Storage/Dipping Sample 1 1.0/1.0/0.50 Na Caseinate/0.75ambient/ambient Sample 2a 0.50/1.0/1.0 Na Caseinate/0.75 20-25°C./ambient Sample 2b 0.50/0.50/1.0 Na Caseinate/0.75 20-25° C./ambientSample 3 1.0/1.0/0.50 Na Caseinate/0.75 16-18° C./ambient Sample 4*1.9/0/0.50 Na Caseinate/0.75 ambient/ambient Sample 5 0/2.1/0.50 NaCaseinate/0.75 ambient/ambient Sample 6 1.0/1.0/0 Na Caseinate/0.75ambient/ambient Sample 7 1.9/0/0 Na Caseinate/0.75 ambient/ambientSample 8 1.0/0.50/0.25 Na Caseinate/0.75 ambient/ambient Sample 91.0/1.0/0.50 DS10/0.75 ambient/ambient Sample 10 1.0/1.0/0.50 ES/0.3ambient/ambient Sample 11 1.0/1.0/0.50 Tween ® 80/0.75 ambient/ambientSample 12** 1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 13***1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 14****1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 15 1.0/1.0/0.50 NaCaseinate/0.75 16-18° C./ambient Sample 16 1.0/1.0/0.50 NaCaseinate/0.75 16-18° C./ambient *Sample 4 compounded latex exhibitedabout 4% coagulation indicating significant precipitation of solids outof the formulation. **The precure temperature for Sample 12 was ambienttemperature (20° C.). ***The precure time for Sample 13 was a period ofabout 2.5 hours (150 minutes). ****The precure time for Sample 14 was aperiod of about 1 hour (60 minutes).

DS10 refers to Rhodacal® DS-10 which comprises sodium dodecylbenzene(branched) available from Rhone-Poulenc, Inc., Dayton, N.J. ES refers toRhodapex ES which comprises sodium lauryl (3) sulfate available fromRhone-Poulenc, Inc., Dayton, N.J. Tween® 80 comprises polysorbate 80 andpolyoxyethylene (20) sorbitan monooleate available from ICI Americas,Inc. (Wilmington, Del.). Unless indicated otherwise, “ambient”temperature was measured as approximately 20° C. Precure temperature andtime for each of Samples 1 through 11 was a temperature of 30° C. for aperiod of approximately 2 hours (120 minutes).

Each of the samples was then evaluated for tensile strength inaccordance with ASTM D 412-98a “Standard Test Methods for VulcanizedRubber and Thermoplastic Elastomers-Tension” (1998) with no exceptionsusing an Instron® testing apparatus. The average tensile strength valuesfor each sample were calculated from averaging five individual samplesper day storage value. The average tensile strength values for each ofthe samples tested are summarized in the following Table 2:

TABLE 2 Tensile Strength Corresponding to Differing Latex StoragePeriods Tensile Strength (psi) @ Compounded Latex Storage Time Sample #Day 0 Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8  1 2698 3648 33983080 — 2651 2548 — 2410  2a — 3768 3640 3839 — — 3441 3541 —  2b — 34983782 3882 — — 3939 3043 —  3 2201 2413 3192 3158 3288 3154 3008 30002909  4 3242 3609 — 3515 3244 3096 2498 — 2464  5 1483 1733 — 2149 15901534 1478 — 1358  6 No tensile measured/sample did not break  7 Notensile measured/sample did not break  8 1018 1063 3051 — 2177 — — 1802—  9 — 2566  914 2843 — — — — — 10 — 1278 2520  839 — — — — — 11 — 24072901 3042 2834 — — — — 12 — 2450 — — — 2374 — 2212 — 13 2544 3213 31812974 2770 — 2393 — — 14 1595 2221 2838 — 2383 — — 1805 — 15 2084 29742452 3497 3312 3075 3056 2979 2968 16 2194 2904 3064 3110 3170 3002 28852902 2746

As can be seen from the above data, synthetic elastomeric polyisoprenesamples prepared in accordance with the invention can exhibitsignificantly elevated tensile strengths of about 3000 psi, even afterusing compounded latex which has been stored for periods of at least 5days and lasting up to about seven (7) days. In general, the besttensile strength values per day latex storage were obtained using thecombination of the three preferred accelerator compounds (ZDEC/ZMBT/DPG)and preferred phr ratios (0.50 to 1.00/0.50 to 1.00/0.50 to 1.00 phr),as well as the preferred stabilizer, sodium caseinate. Samples preparedwithout one of the three preferred accelerator compounds exhibitedsignificantly lower tensile strength values, as can be seen from Samples4, 5, 6 and 7. Based on the results of testing of samples 6 and 7, thetensile strength values for these samples failed to meet minimum FDAregulatory standards required for elastomeric materials to be used forsurgeon's gloves, which is set at about 2479 psi.

Samples 3, 15 and 16 were prepared from compounded latex comprising thepreferred accelerator composition ZDEC/ZMBT/DPG in a 1.0/1.0/0.50 phrratio, with sodium caseinate as the stabilizer, and stored attemperatures ranging from about 16° C. to about 18° C. as seen inTable 1. As can be seen from Table 2, these samples exhibited thehighest combination of storage longevity in relation to high tensilestrength values.

Sample 4, which was prepared from an accelerator composition without thethiazole compound, demonstrated high tensile strength values. Thiscompounded latex, however, exhibited an undesirable amount ofprecipitation of solids out of the composition.

Based on the tensile data that was compiled for samples 9, 10 and 11,the use of stabilizers in the compounded latex other than preferredstabilizer sodium caseinate resulted in samples with significantlyreduced tensile strength per given storage period when compared tosamples 1 through 3 and 15 and 16, for example.

Samples 12, 13 and 14 were prepared from latex compositions undervarying pre-cure time and temperature parameters. As can be seen fromTable 2, deviations in pre-cure temperature and time conditions can alsosignificantly effect the physical properties of the resulting materialas well.

INDUSTRIAL APPLICABILITY

The invention is useful in manufacturing process for elastomericarticles composed of polyisoprene. The invention affords the ability toproduce synthetic polyisoprene articles which closely mimic the physicalproperties of elastomeric articles made from natural rubber latex. Theinvention can be advantageously incorporated into the manufacturing ofsurgical gloves, condoms, probe covers, dental dams, finger cots,catheters, and the like.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it should be understoodthat many variations and modifications can be made while remainingwithin the spirit or scope of the invention as defined by the claims setforth below.

1. A polyisoprene latex composition comprising: a dithiocarbamatecompound; a thiazole compound; a guanidine compound; and a stabilizer.2. The latex composition of claim 1 wherein the latex compositioncomprises: zinc diethyldithiocarbamate; zinc 2-mercaptobenzothiazole;diphenyl guanidine; and sodium caseinate.
 3. A synthetic elastomericarticle consisting of a composition comprising a polyisoprene, whereinpolyisoprene is the only polymer in the composition, and wherein thearticle has a tensile strength of greater than about 3000 psi asmeasured in accordance with ASTM D412, said article being prepared by aprocess comprising the steps of: a) preparing a polyisoprene latexcomposition comprising a polymer consisting of isoprene monomers,wherein the polyisoprene latex composition comprises polyisoprene, anaccelerator composition and a stabilizer, said accelerator compositioncomprising a dithiocarbamate, a thiazole compound and a guanidinecompound; b) dipping a former into said polyisoprene latex composition;c) curing said polyisoprene latex composition on said former; and d)leaching said cured polyisoprene latex composition in a water leachingtank.
 4. The article of claim 3, wherein the article is a glove.
 5. Thearticle of claim 3, wherein the article is a condom.
 6. The article ofclaim 3, wherein the article is a probe cover.
 7. The article of claim3, wherein the article is a catheter.
 8. The article of claim 3, whereinsaid accelerator composition comprises: zinc diethyldithiocarbamate;zinc 2-mercaptobenzothiazole; and diphenyl guanidine.
 9. The article ofclaim 3, wherein said stabilizer comprises a milk protein salt that isoptionally present in the resulting synthetic elastomeric article. 10.The article of claim 3, wherein said stabilizer comprises sodiumcaseinate that is optionally present in the resulting syntheticelastomeric article.
 11. The article of claim 3, wherein saidaccelerator composition comprises: a) a dithiocarbamate compound, in anamount ranging from 0.50 phr to about 1.00 phr per 100.00 phrpolyisoprene of the compounded latex composition; b) a thiazolecompound, in an amount ranging from 0.50 phr to about 1.00 phr per 100.0phr polyisoprene of the compounded latex composition; and c) guanidinecompound, in an amount ranging from 0.50 phr to about 1.00 phr per 100.0phr polyisoprene of the compounded latex composition.
 12. A gloveconsisting of a composition comprising a polyisoprene, whereinpolyisoprene is the only polymer in the composition, and wherein theglove has a tensile strength of greater than 3000 psi as measured inaccordance with ASTM D412, said glove being prepared from a polyisoprenelatex composition comprising a polymer consisting of isoprene monomers,wherein the polyisoprene latex composition comprises polyisoprene, anaccelerator composition comprising a dithiocarbamate compound, athiazole compound, and a guanidine compound.
 13. The glove of claim 12,wherein said polyisoprene latex composition further comprises a milkprotein salt that is optionally present in the resulting glove.
 14. Theglove of claim 13, wherein said polyisoprene latex composition is stableto storage for up to at least 7 days prior to its use in the dipping andcuring process.
 15. The glove of claim 13, wherein said milk proteinsalt is sodium caseinate.
 16. A synthetic elastomeric article consistingof a composition comprising a polyisoprene, wherein polyisoprene is theonly polymer in the composition, said article being prepared by aprocess comprising the steps of: a) preparing a polyisoprene latexcomposition comprising a polymer consisting of isoprene monomers,wherein the polyisoprene latex composition comprises polyisoprene, anaccelerator composition and a stabilizer, said accelerator compositioncomprising a dithiocarbamate compound, a thiazole compound and aguanidine compound; b) dipping a former into said compounded latexcomposition; and c) curing said compounded latex composition on saidformer.
 17. A glove consisting of a composition comprising apolyisoprene, wherein polyisoprene is the only polymer in thecomposition, said glove being prepared from a polyisoprene latexcomposition comprising a polymer consisting of isoprene monomers,wherein the polyisoprene latex composition comprises polyisoprene, anaccelerator composition comprising a dithiocarbamate compound, athiazole compound, and a guanidine compound.